The present invention relates to a rubber composition using deproteinized natural rubber and a pneumatic tire using the rubber composition for a tire tread.
Natural rubber has excellent raw rubber strength (green strength) compared to synthetic rubber and is excellent in processability. Also, natural rubber has high mechanical strength and excellent abrasion resistance as vulcanized rubber and therefore is frequently used in large tires such as truck/bus tires. However, in recent years, from the viewpoint of environmental problems such as tightening regulations for resource conservation and prevention of carbon dioxide exhaust, fuel efficiency of large tires is considered important and improving fuel efficiency without losing abrasion resistance, which is an excellent property of natural rubber, is an urgent task.
Also, in natural rubber, it is known that non-rubber components such as protein and lipid that are present in natural rubber cause molecules to intertwine with one another, thereby causing gelation. When gelation occurs, there is the problem that viscosity of the rubber increases and processability becomes poor. Usually, in order to improve processability of natural rubber, the method of lowering the molecular weight by masticating with a mixing mill or an internal mixer is used. However, mastication randomly cuts molecular chains and as a result, causes fuel efficiency to become poor. On the other hand, the method of removing protein, which is one of the factors that cause gelation, has been suggested as a method for inhibiting gelation. For example, the method of removing non-rubber components such as protein by centrifuging latex is known (see JP-A-6-56902, JP-A-8-143606, JP-A-11-71408, JP-A-2000-198801, JP-A-6-329838). However, the natural rubber obtained by these methods has the problem that as a result of removing non-rubber components and gel content, molecular weight decreases and raw rubber strength decreases.